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Abstract

The impact of upper state lifetime and spatial hole burning on pulse shape and stability in actively mode locked QCLs is investigated by numerical simulations. It is shown that an extended upper state lifetime is necessary to achieve stable isolated pulse formation per roundtrip. Spatial hole burning helps to reduce the pulse duration by supporting broadband multimode lasing, but introduces pulse instabilities which eventually lead to strongly structured pulse shapes that further degrade with increased pumping. At high pumping levels gain saturation and recovery between pulses leads to suppression of mode locking. In the absence of spatial hole burning the laser approaches single-mode lasing, while in the presence of spatial hole burning the mode locking becomes unstable and the laser dynamics does not reach a steady state anymore.

a) Device used in the experiment, b) Modulation scheme for the actively mode locked QCL. The cavity is in total 2.6 mm long and the short modulator section at the beginning of the cavity is 240 μm long.

Intensity profile (blue line) and total inversion (red line) along the cavity for DC pumping p = 1.1 and AC amplitude m = 5 a) when the pulse is reflected off the left facet and propagates to the right b) when the pulse is in the middle of the cavity and propagates to the right c) when the pulse is reflected off the right facet and propagates to the left.

Intensity profile (blue line) and total inversion (red line) along the cavity for DC pumping p = 2 and AC amplitude m = 5 a) when the pulse is reflected off the left facet and propagates to the right b) when the pulse is in the middle of the cavity and propagates to the right c) when the pulse is reflected off the right facet and propagates to the left.

Interferometric autocorrelation traces (IACs) for modulation with AC amplitude m = 5, and DC pumping level p = 1.1 a) for T1 = 50 ps and Tg = 10 ps b) for T1 = 5 ps and Tg = 2.5 ps. Comparing the two figures with Fig. 13a we see that the structure is more pronounced in these figures due to the stronger SHB.